1. Technical Field
This invention relates to a method for fabricating a composite structure. More particularly, this invention relates to a method of using molded, rigid thermoplastics to create a conformal vacuum bag/temporary tool (mandrel) for fastenerless assembly and curing of composite structures.
2. Background of the Invention
Composite materials comprising a fiber-reinforced resin matrix are often used to fabricate lightweight, high strength parts such as vertical and horizontal stabilizers for aircraft. Optimized designs for these types of parts will typically involve geometries that cause the internal tooling to become trapped upon cure. Fabrication of composite parts with trapped geometries typically includes use of a solid mandrel having an exterior shape generally conforming to the desired interior shape of the part. Uncured composite materials are then laid up on the mandrel and cured by applying heat and pressure according to well-known methods.
To address the problem of mandrel removal, industry presently uses one of two basic mandrel types: 1) segmented metal, or; 2) expendable.
Reusable segmented metal mandrels have been used with a wide variety of geometries and sizes. They are extracted from a cured structure by disassembling the mandrel and removing it piece by piece. These mandrels are very costly and present handling problems due to their extreme weight and complexity, and because they are somewhat damage-prone.
Expendable mandrels are typically made of plaster, water soluble eutectic salts or even eutectic metals. In the case of plaster, it is removed by breaking it away from the part using impact devices after the composite part has been cured in an oven or autoclave. The broken plaster pieces are then discarded at a significant cost to the manufacturer. The use of breakaway plaster is labor intensive, can result in irreparable damage to cured composite parts, and produces large quantities of waste which are costly to dispose of.
The use of eutectic salts or metals may be environmentally hazardous, and although some percentage may be recoverable, recovery is not cost effective in many cases due to contamination and/or degradation of the material. These materials also tend to fuse to, and therefore contaminate the interior surface of a structure, making it necessary to provide a reliable barrier, which also needs to be subsequently removed. These mandrels are heavy, particularly those made with eutectic metals. Salt mandrels are comparatively fragile, therefore the handling of heavy and/or fragile mandrels presents yet another drawback.
All four types are unsuitable for co-bonded structures because they do not allow direct application of outward pressure to the interior surfaces. Instead, pressure is applied only to the outside, requiring the outer surfaces and tooling to move inward. While this method does provide accurate interior mold lines, it is difficult to control outer mold line contours and it becomes equally challenging to maintain distortion-free fiber alignment. The substantial thermal mass associated with these materials presents yet another complication when the time comes to quickly and uniformly heat the structure during cure. This heat-up lag can increase the risk of having to scrap a structure by simply forcing the cure process to go beyond its acceptable, specified parameters.
Expandable elastomeric soft tooling has occasionally been used as part of the bagging envelope in specific types of composite structure fabrication. In these cases, it is used to apply pressure to interior surfaces. One such process utilizes an expandable tool where pressure is applied to the interior of the tooling and expanded to force the uncured composite material to conform and consolidate against the surfaces of an external tool. These mandrels are limited to simple design configurations in which dimensional control of the part""s internal surfaces part is not critical. While this system does address the problem of consolidating internal elements of a composite structure such as those encountered in co-bonded assemblies, its flexible nature cannot provide a rigid backbone upon which these various elements can be located or assembled prior to cure. Complex and cumbersome tooling must therefore be used for that purpose. As a result, non-recurring and recurring costs increase due to the additional tooling required and/or the complications arising from having to extract tooling prior to cure, or from having to bag around these tooling elements. It is difficult for these mandrels to achieve uniform pressure distribution across the laminate during a cure cycle. Finally, elastomeric soft tooling is limited in its ability to be extracted intact from severely trapped areas. It is comparatively expensive, easily damaged and the elastomer itself cannot withstand more than a few cure cycles before having to be replaced.
Traditional bagging systems, such as nylon film, also typically require tooling to support the interior cavities. In addition, these systems are difficult to conform to, and be extracted from, complex geometry. The film has a tendency to bridge over corners, increasing the risk of bag failure considerably during autoclave cure. This usually results in a poorly consolidated laminate which will have to be scrapped. Although tooling is typically added to minimize this risk, it becomes a labor intensive remedy.
Hence, the need exists for a method/tooling material which will facilitate fabrication of trapped geometry composite structure while minimizing the aforementioned costs and risks. At room temperature, it would need to act as a rigid mandrel during lay-up and assembly of a structure. When the time comes to cure that structure, however, it would have to behave in quite a different manner: that of a highly flexible, yet inherently reliable vacuum bag. A tool is needed that will provide support to the composite structure during assembly, consolidate the laminate during cure, and yet, become safely and easily removable from the structure after it is cured. The present invention meets these needs.
In this invention, a method of fabricating composite structure is provided that uses thermoplastics to create conformal vacuum bag/temporary tool. This tool can accurately locate and provides support to at least a portion of elements within a composite structure prior to cure and yet provide improved pressure distribution during the cure by functioning as a high integrity vacuum bag. The thermoplastic conformal tool/vacuum bag is designed to conform or fit closely to internal surfaces or cavities of a composite structure. Such surfaces or cavities would typically consist of a closed (i.e., trapped) geometry. Some, if not all, of the elements in the structure may be uncured when assembled and placed into the cure tool or fixture. Although the conformal tool becomes trapped in such geometry upon cure, it can be easily removed by reheating the cured composite assembly to an intermediate temperature. This softens the material to a point at which it will readily collapse and allow it to be easily withdrawn, regardless of the geometric complexity of the structure.
The method of fabricating a composite structure using a thermoplastic conformal tool/vacuum bag of this invention comprises the steps: of preparing a rigid thermoplastic conformal tool/vacuum bag to dimensions that substantially conform to a surface or a cavity of a composite structure; placing the thermoplastic conformal tool/vacuum bag and uncured composite members in contact with each other to form an assembly; applying external heat and external pressure to the assembly for sufficient time to cure the composite member; and removing the thermoplastic conformal tool/vacuum bag from the assembly, whereby a cured composite structure is fabricated. Also, the volume of the assembly is reduced to form a debulked assembly, if needed.
Elements of the structure are located on the conformal tool according to predetermined geometries and recesses that were previously molded into the tool. External elements and/or tooling are placed around the assembly and covered by a vacuum bag. The external vacuum bag is sealed to the end(s) of the conformal tool(s) Heat and pressure are applied for a sufficient time to cure the assembly. External bagging and tooling are removed, and the cured structure is then reheated to permit collapse and extraction of the softened tool/bag.